Dispersed power supplies, such as a solar light power generation device, a wind power generation device, a fuel cell battery, and a secondary battery, need a power conversion device for a connection to a load and an interconnection with a system. For example, a solar power generation device is configured to be connected to a power conversion device called a Power Conditioning System (PCS) in order to supply desired power to a system.
Present PCSs include, in many cases, a power converter like a DC-DC converter to boost up the voltage and a so-called inverter for DC-AC conversion, and a control unit thereof. In addition, the PCS includes a Maximum power point Tracker (MPPT) control unit that tracks the maximum power point where the output power becomes the maximum during a change in amount of solar radiation time by time.
According to the MPPT, under the weather conditions, etc., that always fluctuate, the value of current×voltage that maximizes the power, i.e., the maximum power point is automatically obtained. According to such an MPPT, a generally adopted scheme to search the maximum power point is a hill climbing method. The hill climbing method is a method which changes the battery voltage by a predetermined step width at a certain time interval, checks the increase or decrease of the output power, and increases or decreases the voltage so as to always increase the output power, thereby searching the maximum power point.
As for the searching technologies relating to the hill climbing method, the following technologies are proposed.
(1) As the primary search, a searching operation is performed at a rough voltage step width set within the certain voltage range. Next, after the neighborhood of the top among the plural mountains is detected, as the secondary search, a searching operation is performed at a narrower voltage step width than that of the primary search (see, for example, Patent Document 1).
(2) After the search within the certain voltage range, an approximated curve is calculated based on the relationship between the generated power and the voltage, and the voltage step width is set in accordance with the curvature of the curved line (see, for example, Patent Document 2).
(3) Across the maximum power point in the current-voltage curved line, the searching operation is divided into the current mode and the voltage mode. In the current mode, the current is changed at a predetermined current step width, and in the voltage mode, the voltage is changed at a predetermined voltage step width (see, for example, Patent Document 3).
(4) The detection error originating from capacitance parasitic in the solar battery is reduced. That is, when the voltage and the current are changed at fast speed, the current-voltage curved line is changed due to the discharging and charging of the capacitance, generating a hysteresis loop. In order to eliminate this effect, the voltage and the current at a time point at which the voltage time-derivative value becomes zero are detected (see, for example, Patent Document 4).